I'm not sure what happened to my site. It just changed the layout and blocked the main(only) user.
Looks like a corrupt cache. If the user 'admin' can not log in you can't of course purge the cache. To do it in the back end, one needs shell access, which you don't get in MoodleCloud. Hope a sysadmin there will see this post soon. Otherwise there must be a ticket system in MoodleCloud.